The corrosion resistance of stainless steel generally increases with the increase of chromium content. The basic principle is that when there is sufficient chromium in the steel, a very thin to dense oxide film is formed on the surface of the steel, which can prevent further oxidation. An oxidizing environment can strengthen this film, while a reducing environment will inevitably destroy this film and cause corrosion of steel.
Corrosion resistance in various environments
atmospheric corrosion
The atmospheric corrosion resistance of stainless steel basically varies with the content of chloride in the atmosphere. Therefore, proximity to marine or other sources of chloride pollution is extremely important for the corrosion of stainless steel. A certain amount of rainwater is only important when it affects the chloride concentration on the steel surface.
1Cr13, 1Cr17, and austenitic stainless steel in rural environments can adapt to various applications without significant changes in appearance. Therefore, stainless steel exposed for use in rural areas can be selected based on price, market supply, mechanical properties, manufacturing and processing properties, and appearance.
In industrial environments free from chloride pollution, 1Cr17 and austenitic stainless steel can work for a long time and remain basically rust free. It may form a fouling film on the surface, but when the fouling film is removed, it still maintains its original bright appearance. In industrial environments with chlorides, stainless steel corrosion can occur.
In the marine environment, 1Cr13 and 1Cr17 stainless steel will form a thin rust film in a short period of time, but it will not cause significant dimensional changes. Austenitic stainless steel such as 1Cr17Ni7, 1Cr18Ni9, and 0Cr18Ni9 may exhibit some rust when exposed to the marine environment. Rust is usually shallow and can be easily removed. 0Cr17Ni12M02 molybdenum containing stainless steel is basically corrosion-resistant in marine environments.
In addition to atmospheric conditions, there are two other factors that affect the atmospheric corrosion resistance of stainless steel. Namely, surface condition and manufacturing process. The level of precision machining affects the corrosion resistance of stainless steel in environments with chlorides. Matte surfaces (rough surfaces) are very sensitive to corrosion. Normal industrial precision machined surfaces are less sensitive to corrosion. The level of surface finishing also affects the removal of dirt and rust. It is easy to remove dirt and rust from high-precision machined surfaces, but it is difficult to remove from matte surfaces. For matte surfaces, frequent cleaning is necessary to maintain their original surface state.
freshwater
Fresh water can be defined as water from rivers, lakes, ponds or wells, regardless of acidity, salinity or brackish.
The corrosiveness of fresh water is influenced by the pH value, oxygen content, and scaling tendency of the water. Fouling (hard) water. Its corrosiveness is mainly determined by the amount and type of scale formed on the metal surface. The formation of this scale is due to the presence of minerals and temperature. Non scaling (soft) water, which is generally more corrosive than hard water. The corrosiveness can be reduced by increasing the pH value or reducing the oxygen content.
1Cr13 stainless steel is significantly more resistant to freshwater corrosion than carbon steel, and has excellent characteristics when used in freshwater. This type of steel is widely used for purposes such as docks and dams that require high strength and corrosion resistance. However, consideration should be given to certain situations. 1Cr13 may be sensitive to moderate pitting in freshwater, but pitting can be completely avoided using cathodic protection methods. 1Cr17 and austenitic stainless steel are almost completely resistant to freshwater corrosion at room temperature (ambient temperature).
Acidic water
Acidic water refers to the polluted natural water extracted from ore and coal leaching, which is much more corrosive than natural fresh water due to its strong acidity., Due to the leaching effect of water on sulfides in ores and coal, acidic water usually contains a large amount of free sulfuric acid. In addition, this water contains a large amount of iron sulfate, which has a significant effect on the corrosion of carbon steel.
Carbon steel equipment affected by acidic water usually corrodes quickly. The results of experiments conducted on various materials affected by acidic river water indicate that austenitic stainless steel has high corrosion resistance in this environment.
Austenitic stainless steel has excellent corrosion resistance in fresh water and acidic river water, especially its corrosion film has less resistance to heat conduction, so stainless steel pipes are widely used in heat exchange applications.
Saline water
The corrosion characteristic of saline water is often in the form of pitting corrosion. For stainless steel, it is largely due to the localized damage of the corrosion resistant passivation film caused by saline water. The other reason for the pitting corrosion of these steels is that the concentration difference battery can be formed by the presence of organic compounds in the water and other seawater attached to the stainless steel equipment. Once formed, these batteries are very active and cause significant corrosion and pitting. In the case of high-speed flow of saline water, such as the impeller of a pump, the corrosion of austenitic stainless steel is usually very small.
For condensers using stainless steel pipes, it is necessary to maintain a water flow rate greater than 1.5m/s to minimize the aggregation of organic matter and other solids in seawater in the pipes. It is best to reduce gaps and use thick walled components in the design of stainless steel equipment for treating saline water.
soil
The metals buried in the soil are in a complex state that changes at any time, depending on weather and other factors. Practice has shown that austenitic stainless steel generally has excellent resistance to most soil corrosion, while 1Cr13 and 1Cr17 exhibit pitting corrosion in many soils. 0Cr17Ni12Mo0 stainless steel can completely resist pitting corrosion in all soil tests.
nitric acid
Ferritic stainless steel and austenitic stainless steel containing no less than 14% chromium have excellent resistance to nitric acid corrosion. 1Cr17 stainless steel has been widely used in processing equipment in nitric acid factories. However, due to the good formability and weldability of 0Cr18Ni9, it has largely replaced 1Cr17 stainless steel in the aforementioned applications
The nitric acid corrosion resistance of other austenitic stainless steels is similar to that of 0Cr18Ni9. 0Cr17 stainless steel usually has a slightly higher corrosion rate than 0Cr18Ni9, and higher temperatures and concentrations have significant harmful effects on it.
If the heat treatment of the steel is not appropriate, hot nitric acid will cause intergranular corrosion of austenitic and ferritic stainless steel. Therefore, appropriate heat treatment can be used to prevent this type of corrosion, or stainless steel that is resistant to this type of corrosion can be used.
sulphuric acid
Standard stainless steel grades are rarely used in sulfuric acid solutions because their usable range is very narrow. At room temperature, 0Cr17Ni12Mo2 stainless steel (the most resistant standard grade to sulfuric acid corrosion) has a sulfuric acid concentration of less than 15%. When it is greater than 85%, it is corrosion-resistant. However, in higher concentration ranges, carbon steel is usually used. Generally, martensite and ferritic stainless steels are not resistant to sulfuric acid solution.
Just like in the case of nitric acid, if stainless steel is not properly treated, sulfuric acid can cause intergranular corrosion. For welded structures that cannot undergo heat treatment after welding, low-carbon grades 00Cr19Ni10 or 00Cr17Ni14M02, or stabilized grades 0Cr18Ni11Ti or 0Cr18Ni11Nb stainless steel should be used
phosphoric acid
Austenitic stainless steel has good corrosion resistance to phosphoric acid solution and is widely used in phosphoric acid production and treatment equipment. It has effective corrosion resistance at various concentrations up to 107 ℃. Equipment made of 0Cr17Ni12M02 stainless steel can effectively treat (up to phosphoric acid) at temperatures up to approximately 95 ℃, exceeding 100% H3p04.
It should be noted that trace impurities such as fluoride or chloride salts are sometimes present in phosphoric acid produced by wet processes. The presence of these halides in acids may have harmful effects on the corrosion resistance of stainless steel.
The phosphoric acid corrosion resistance of martensite and ferritic stainless steels is significantly worse than that of austenitic stainless steels, so they are generally not used for this acid.
hydrochloric acid
Even at room temperature, various concentrations of hydrochloric acid solutions quickly corrode stainless steel. Therefore, it is not possible to use stainless steel in this type of acid.
Other inorganic acids
Austenitic stainless steel generally has good corrosion resistance to boric acid, carbonic acid, chloric acid, and chromic acid at almost all concentrations and temperatures, except for 100% chloric acid. The corrosion resistance of 1Cr13 and 1Cr17 stainless steel to chromic acid is significantly lower than that of austenitic stainless steel, but they have relatively good resistance to boric acid and carbonate corrosion.
acetic acid
Austenitic stainless steels generally have excellent resistance to acetic acid corrosion, while martensite and ferritic stainless steels are inappropriate for most applications of acetic acid corrosion resistance. Austenitic stainless steel can fully withstand the corrosion of various concentrations of acetic acid at room temperature. At higher temperatures, 0Cr17Ni12Mo2 and 0Cr19Ni13M03 have better resistance to acetic acid corrosion than other Austenitic stainless steels.
formic acid
At room temperature, formic acid can be completely treated with any austenitic stainless steel. However, when it is hot formic acid, it can quickly corrode stainless steel without molybdenum, so 0Cr17Ni12M02 and 0Cr19Ni13M03 need to be used. Formic acid will quickly corrode martensite and ferritic stainless steel at various temperatures.
oxalate
In general, stainless steel has good resistance to oxalic acid corrosion at room temperature and a maximum concentration of at least 50%. However, at higher temperatures, oxalic acid solution can cause significant corrosion to all stainless steels, just as it does at room temperature and a concentration of 100%.
lactic acid
0Cr18Ni9 stainless steel can be used for lactic acid storage equipment at temperatures up to approximately 38 ℃. At higher temperatures, molybdenum free austenitic stainless steel produces pitting corrosion, so 0Cr17Ni12M02 and 0Cr19Ni13M03 are preferred. Generally speaking, martensite and ferritic stainless steels have low resistance to lactic acid corrosion.
alkali
Stainless steel usually has good resistance to weak alkali corrosion, such as ammonium hydroxide. For strong alkalis such as sodium hydroxide and potassium hydroxide, austenitic stainless steel has good corrosion resistance at temperatures up to about 105 ℃ and concentrations up to about 50%. At higher temperatures and concentrations, the corrosion rate may become significant. When the temperature is above the atmospheric boiling point (and slightly lower temperature, close to 50% concentration), austenitic stainless steel will undergo stress corrosion cracking.
Hydrochloric acid solution
Except for halide solutions under certain conditions, stainless steel generally has excellent corrosion resistance to hydrochloric acid solutions. For acidic salts, the corrosion resistance of stainless steel is inevitably affected to some extent by the special acid formed by saltwater hydrolysis. For acidic salt solutions at higher temperatures, molybdenum containing austenitic stainless steel (0Cr17Ni12Mo2 and 0Cr19Ni13Mo3) typically has better corrosion resistance than other grades of stainless steel.
When stainless steel is used in halide solutions, especially chloride solutions, it should be considered that even if the corrosion rate is generally low, pitting and/or stress corrosion cracks may still occur under certain conditions. Although there are many cases where stainless steel can achieve excellent results in the presence of chlorides (such as food processing equipment and seawater flowing at relatively low temperature conditions), various uses need to be considered separately. The occurrence of pitting or stress corrosion cracks depends on many factors such as the environment and equipment design and operation.